I have a 94 Chevy Corsica it's getting about 19 mpg highway when it should be getting a lot better I replaced spark plugs fuel filter before I replace the O2 sensor wondering what the problem could be

Don't know where you live but if you're trying to compare mileage that you were getting over the summer to the mileage this time of year, it will always be bad. Cars always get worse mileage during the winter months. They also change the gas to a "winter blend" which is worse for mileage than the summer blend. Try checking your mileage again when the weather gets warmer. You will see a difference. Check your air filter. That never helps running a dirty one.

That should be the 3.1 engine. If you have had better milage and it has dropped without codes it is a pain to find the cause on these cars. The best method to chase down the issue is to read the live data in the ECM to see which sensor is NFG. Since this is a '94 it is OBD1 and the scanner market is fading away. I use a ALDL cable and my laptop (about 70 bucks for the cable) to read the data. But for now I would get a cheep manual and check the other sensors. The ECT is always a good one, if you have a low temp reading the ECM will not enter closed loop and trim your fuel with the 02 sensor? You can also check the EGR valve? If it's flooding or rich check your fuel pressure regulator and injectors?